LDLs are a type of lipoprotein that transports cholesterol. It has been elucidated medically that LDL cholesterol, which is a complex of an LDL and cholesterol, is one of the causative substances of arteriosclerosis. The measurement of LDL cholesterol levels newly was added in April, 2008, as a test item for medical examinations according to the Japanese Industrial Safety and Health Act.
An example of a method for measuring LDL cholesterol is to separate LDLs from other lipoproteins (e.g., high-density lipoproteins (HDLs), very low-density lipoproteins (VLDLs), etc.) by ultracentrifugation and then measure only the LDL cholesterol using an enzyme. Another example is a method in which LDLs are separated by electrophoresis, lipids then are stained and the extent of color development thereof is measured. Although these methods are generally-used measurement methods, their operation is troublesome and it is difficult to process a large number of specimens. Therefore, these methods are not applicable to clinical examinations or the like.
Thus, a variety of methods for measuring LDL cholesterol for use in a liquid system in which an enzyme reagent is used have been developed, and some are in use. Since such measurement methods that use an enzyme reagent can be performed using an autoanalyzer, they are of use in clinical examinations where a large number of specimens need to be processed. Examples of such measurement methods that use an enzyme reagent include a direct method in which LDL cholesterol itself is directly measured, an elimination method in which types of cholesterol other than LDL cholesterol (non-LDL cholesterol) are subjected to a reaction and eliminated in a first reaction and the remaining LDL cholesterol is subjected to a reaction in a second reaction to measure an LDL cholesterol level, and like methods.
An example of the direct method is a method in which a direct measurement of LDL cholesterol is performed using an enzyme reagent whose specificity to LDL cholesterol is enhanced by cyclodextrin and a surfactant (Patent Document 1). Examples of the elimination method include methods in which a reaction between LDL cholesterol and an enzyme reagent is inhibited, or the reactivity of an enzyme reagent with non-LDL cholesterol is enhanced, using a polyanion and a divalent metal ion (Patent Document 2), a polyanion (Patent Document 3), calixarene sulfate (Patent Document 4) or amine (Patent Document 5). Liquid-system reagents for use in the direct method and the elimination method are available from different manufacturers.
Patent Document 1: JP 3091230B
Patent Document 2: JP 3193634B
Patent Document 3: JP 3767232B
Patent Document 4: JP 3822340B
Patent Document 5: JP 3058602B